Preheat the oven to 190°C.
Heat 30ml of olive oil in a wide, lidded pan and add the chard ribs.
Cover and cook over a low heat for four minutes.
Add sliced chard leaves, cover, and cook for five minutes until tender.
Season chard with salt and pepper and set aside.
Blend olives, anchovies, tomatoes, capers, parsley, and 30ml olive oil into a paste.
Lightly oil a 30cm ovenproof dish.
Layer chard and sliced potatoes in the dish, spreading olive paste between layers.
Finish with a final layer of chard and season with pepper.
Mix Gruyère and Parmesan and sprinkle over the gratin.
Drizzle the remaining 15ml of olive oil over the top.
Bake for 25 to 30 minutes until the cheese is browned and bubbling.